HP 3400B, User'S And Service Manual

Page: 44 / 77

Chapter
3
Theory of Operation
Post Attenuator
Post Attenuator
The
post attenuator is a resistive divider which attenuates the ac input
signal while maintaining a low impedance output for the following
amplification stages. The ac input signal is applied to a precision
resistive voltage divider. These resistors are arranged to give six ranges
of attenuation at
10 dB
per range. The six ranges of the post attenuator
combined with the two ranges of the input attenuator make up the
12
ranges of attenuation (1 mV to
300
V).
Wideband Amplifier
The wideband amplifier provides constant gain to the ac signal being
measured over the entire frequency range of the HP
3400B.
The
wideband amplification required to drive the thermocouple heater is
provided in two gain stages. The first stage has a gain of
36 dB
and
consists of a differential transistor pair Q301 and Q302 driving U301.
This gain stage also includes the capacitor feedback adjustment (C304)
to adjust the voltmeter's frequency response at
20
MHz. The second
stage has a gain of
14 dB
to the thermocouple heater and consists of U302.
The combined gain of
50 dB
amplifies the
1
mV RMS full-scale input
signal to
316
mV RMS applied to the thermocouple heater. The nominal
resistance of the heater is 90n and there is
3.5
rnA RMS of current
flowing at full-scale input signal.
Protection against excess current to the thermocouple heater is
provided by controlling the power supply voltages to the heater drive op
amp (U302). The output from U302 is peak-detected by CR306 and CR307
and is then averaged with an RC network. The average de level is
compared with a reference by comparators U304A and U304B. The
comparator output lowers the supply voltage via Q303 and Q304 when
the average de level exceeds the reference level. The average dc level of
the input signal allows for
10-to-l
crest factor signals to be applied to
the heater at proper levels without being attenuated.
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